1. Field of the Invention
The present invention relates to a sheet stacking device and an image forming apparatus including the same.
2. Description of the Related Art
A sheet stacking device such as an exit tray used for stacking sheets ejected from an image forming apparatus is required to have an adequate stackability, namely a stacking ability enough to stack ejected sheets tidily on the sheet stacking section, thereby eliminating a need for a user to align the sheet bundle after taking out the stacked sheets therefrom. If the sheet stacking device has an insufficient staking ability, the user is required to align the sheet bundle taken out from the sheet stacking section by hand, thereby requiring the user to perform extra operations.
On the other hand, in recent years, there has been increasingly a need for higher-speed image forming apparatuses. In order to address the need, apparatuses with higher printing speeds have been provided. For example, although apparatuses with printing speeds of 60 sheets per minute (in cases of transferring A4 sheets in the lateral direction) or more have been conventionally regarded as high-speed apparatuses, in recent years apparatuses with printing speeds of 80 sheets per minute or more have been regarded as high-speed apparatuses. Furthermore, even apparatuses with printing speeds of 100 sheets per minute or more have been developed. Such high-speed apparatuses have tendency to increase the sheet ejecting speeds in ejecting sheets from the apparatuses. The increase of the ejecting speeds increases the difficulty in ensuring adequate stackability with the sheet stacking sections.
Therefore, for example, there has been employed a method of inclining an exit tray such that the side of the exit tray farther from an exit portion is higher, for allowing the top end of ejected sheets in an ejecting direction to come into contact with the exit tray rapidly. This can apply a braking force to the sheets with the frictional force between the exit tray and the sheets and also can rapidly brake the sheets after the sheets are separated at their rear end from the exit roller to lose propulsion forces. The rear end of sheets drop due to their weights (for example, refer to Japanese Unexamined Patent Publication No. HEI 11(1999)-180615).
Further, there has been known a method of inclining an exit tray such that its side closer to an exit port is higher for aligning the front end of sheets (for example, refer to Japanese Unexamined Patent Publication No. HEI 7(1995)-242361). However, with this method, in cases where ejected sheets have more than one size, for example, in cases where there are sheets having a greater length in the ejecting direction (larger-size sheets) and sheets having a smaller length in the ejecting direction (smaller-size sheets) and also there is a significant length difference therebetween, the rear end of a previously ejected smaller-size sheet drops to a position farther than the drop position of a subsequently ejected smaller-size sheet, which may inconveniently cause the edge of the subsequent sheet to crawl under the previous sheet. This changes the order of ejected sheets, even though a user does not desire that.
As described above, high-speed image forming apparatuses have sheet transfer speeds higher than conventional sheet transfer speeds. Accordingly, it is more difficult to ensure an adequate stackability by braking sheets through the frictional force between the sheet edge and the exit tray, than in medium and lower speed apparatuses.
Furthermore, in some cases, it is difficult to ensure an adequate stackability by relying only on the sheet propulsive force caused by the exit section and the effect of the gravity, in high-speed apparatuses. There has been a need for a method of controlling the behavior of ejected sheets, according to the types of sheets such as the sizes and the weights of the sheets.